US20090128735A1 - Backlight systems for liquid crystal displays - Google Patents

Abstract

A backlight system for a liquid crystal display includes a substantially planar, refractive waveguide having a first major face and a second major face opposite the first major face. The waveguide includes a viewable region corresponding to a viewable area of the liquid crystal display. The system further includes a light source positioned proximate to the second major face and within the viewing region for producing light. An injection feature is proximate to one or more of the second major face and the first major face and within the viewing region to optically couple the light into the waveguide such that the light becomes waveguided light. A plurality of extraction features is proximate to one or more of the second major face and the first major face and within the viewing region to optically couple the waveguided light out of the waveguide.

Description

TECHNICAL FIELD
• The present invention generally relates to the field of liquid crystal displays (LCDs), and more particularly to direct backlight systems of LCDs.
BACKGROUND
• Liquid crystal display (LCD) monitors are replacing traditional cathode ray tube (CRT) monitors in many applications because of their lighter weight and superior performance. In a typical LCD, a backlight system is placed behind an LCD panel to illuminate the LCD for viewing by a user. An array of light emitting diodes (LEDs) is used as the light source of the backlight system, although other sources of illumination can be provided.
• Conventional backlight systems typically fall into one of the following two categories: direct backlight systems or edge backlight systems. A direct backlight system typically has a light source directly behind the LCD panel with an integrating cavity therebetween that enables mixing of the light from the light source, thereby improving the uniformity of the display. Conventional direct backlights can be problematic, however, in that the cavity can result in an undesirable added thickness. Edge backlight systems include light sources located at the edge of a waveguide (or “light pipe” or “light guide”) placed behind the LCD panel. The light travels from the edge of the light guide until it is deflected towards the LCD panel. Although conventional edge backlight systems may be thinner than conventional direct backlight systems, such displays often fail to provide sufficient luminescence (or “brightness”) for certain applications because the number of light sources is greatly reduced and because the light must propagate throughout the entire light guide from the edge of the display.
• Accordingly, it is desirable to provide an improved backlight system for LCDs. In addition, it is desirable to provide a more compact backlight system with uniform luminescence. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.
BRIEF SUMMARY
• In accordance with an exemplary embodiment, a backlight system for a liquid crystal display includes a substantially planar, refractive waveguide having a first major face and a second major face opposite the first major face. The waveguide includes a viewable region corresponding to a viewable area of the liquid crystal display. The system further includes a light source positioned proximate to the second major face and within the viewing region for producing light. An injection feature is proximate to one or more of the second major face and the first major face and within the viewing region to optically couple the light into the waveguide such that the light becomes waveguided light. A plurality of extraction features is proximate to one or more of the second major face and the first major face and within the viewing region to optically couple the waveguided light out of the waveguide.
• In accordance with another exemplary embodiment, a backlight system for a liquid crystal display includes a substantially planar waveguide including a first major face and a second major face opposite the first major face. The waveguide includes a viewable region corresponding to a viewable area of the liquid crystal display. A light source is positioned proximate to the second major face and within the viewing region for producing light, and an injection feature is positioned proximate to at least one of the second major face and the first major face and within the viewing region to optically couple the light into the waveguide such that the light becomes waveguided light. A plurality of extraction features is proximate to at least one of the second major face and the first major face and within the viewing region to optically couple the waveguided light out of the waveguide. The plurality of extraction features has an extraction density that varies.
• In accordance with yet another exemplary embodiment, a liquid crystal display (LCD) includes an LCD panel having a plurality of pixels and a backlight system coupled to and illuminating the pixels to form an image. The backlight system includes a substantially planar dielectric waveguide including a first major face and a second major face opposite the first major face. The waveguide includes a viewable region corresponding to a viewable area of the liquid crystal display. A light source is positioned proximate to the second major face and within the viewing region for producing light. An injection feature is within the viewing region to optically couple the light into the waveguide via refraction such that the light becomes waveguided light. A plurality of extraction features with an extraction density that varies is within the viewing region to optically couple the waveguided light out of the waveguide.
BRIEF DESCRIPTION OF THE DRAWINGS
• The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and wherein:
• FIG. 1 is a cross-sectional view of an exemplary liquid crystal display (LCD);
• FIGS. 2-22 are views of several exemplary injection features and light sources;
• FIGS. 23 and 24 are cross-sectional views of exemplary backlight systems;
• FIG. 25 is a graph illustrating the spread function of the backlight systems ofFIGS. 23 and 24;
• FIGS. 26 and 27 are planar views of exemplary backlight systems;
• FIGS. 28-31 are cross-sectional views of several exemplary extraction features; and
• FIGS. 32-36 are views of backlight systems with light sources having differing spectral or color characteristics.
DETAILED DESCRIPTION
• The following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background or the following detailed description.
• Exemplary embodiments discussed below provide liquid crystal displays (LCDs) having waveguides with injection features that refract a majority of light from light sources into the waveguide such that the light is contained within the waveguide via total internal reflection (TIR) until extracted by extraction features. Other embodiments include waveguides having extraction features with varying extraction densities. The disclosed embodiments provide a compact backlight system with enhanced lateral spreading, mixing, and luminance.
• FIG. 1 is a cross sectional view of anexemplary LCD100. TheLCD100 includes adirect backlight system104 coupled to anLCD panel102. During operation, thebacklight system104 providesoutput light132 to enable a viewer to view pixel patterns on theLCD panel102 that form an image. TheLCD100 can be used in any display application, including avionic displays.
• In atypical LCD panel102, there is an active matrix array of many thousands of pixel structures. Although not described in greater detail for brevity, theLCD panel102 may include, in one exemplary embodiment, any addressing structure such as a structure that includes thin film transistors processed onto a lower glass plate, cells of liquid crystal material, a common electrode adjacent to the liquid crystal material, color filters processed onto an upper glass plate, and a pair of appropriately oriented linear polarizing films. If desired, an optionaltransmissive diffuser103 may be included withLCD panel102 to further blend andhomogenize output light132. An air gap may be included between thediffuser103 and theLCD panel102. Any light reflected bydiffuser103 will be returned to the backlight for another chance to be scattered, deflected or reflected by the various backlight components before rejoiningoutput light132. Other enhancement films, such as prismatic or lenticular films, reflective or scattering polarizer films, and various types of diffusion films, may also be provided on or adjacent to theLCD panel102 in the path ofoutput light132.
• Generally, thebacklight system104 includes aviewable region105 extending in front and behind thebacklight system104 that corresponds to an area of theLCD100 viewed by a viewer. Thebacklight system104 includes a unitary,refractive waveguide106 formed from a transparent optical material such as glass, acrylic, polycarbonate, transparent polymers, or similar materials. Waveguide106 has one ormore edges112 and twomajor faces108 and110 that are substantially parallel to each other. Thebacklight system104 further includes one ormore light sources116, such as light emitting diodes (LEDs) or other light sources, distributed across theviewing region105. Thelight sources116 are optically coupled to thewaveguide106 by injection features114 distributed across theviewing region105 such that light (e.g., ray124) from thelight sources116 enters thewaveguide106 via refraction.
• As described in further detail below, the light is effectively confined within thewaveguide106 until reaching anextraction feature128 that directs light out of thewaveguide106 for illumination of theLCD panel102. In alternate embodiments, some of thelight sources116, injection features114 and/orextraction features128 are located out of theviewing region105. Thebacklight system104 can further include areflective layer136 behind thewaveguide106 and behind or adjacent thelight sources116.Reflective layer136 serves to redirect rays which happen to be extracted in the opposite direction fromoutput light132, or are otherwise aimed away fromLCD panel102.
• Thewaveguide106 is characterized as being a unitary refractive structure in that substantially all of its distributed substructures, specifically itsmajor faces108,110, injection features114 and extraction features128, are refractive in nature, comprising refractive materials and interfaces, for example clear plastic and air. Light rays incident on refractive interfaces follow well-characterized properties of transmission, reflection or total internal reflection (TIR), depending upon the refractive indices and angles of incidence. Certain other embodiments, described below, may not meet the strict requirements of a “unitary waveguide” in that they may include reflective mirrors, pigments, volume diffusers or other substructures not easily characterized by the laws of refraction. Some will still however contain unitary injection features or unitary extraction features, depending upon the detailed design and constituent structures.
• In the depicted embodiment ofFIG. 1, the injection features114 are conical and appropriately designed, formed, and polished to effectively hide the associatedlight source116 from direct visibility by injecting substantially all of the light. Additional examples of injection features will be described below, each of which can offer potential advantages such as ease of fabrication, support of different light source topologies, or use of more efficient or environmentally suitable optical materials.
• In the depicted embodiment ofFIG. 1, the extraction features128 are conical, and extract most or all of the waveguided light from thewaveguide106. The extraction features128 can be varied as a function of number, size, geometry, and position from the injection features114, and position relative to each other. These parameters result in a set of extraction features128 with a given extraction density, which represents the amount or fraction of light extracted from the waveguide over a given area. Interference with the TIR of the waveguided light is one exemplary extraction mechanism. This can be accomplished by either deflecting wave-guided light rays or by localized deviations in the waveguide surface. The extraction features128 depicted inFIG. 1 and discussed below can utilize one or both of these mechanisms to cause light to be extracted fromwaveguide106.
• In one embodiment, injection features114 can also function as extraction features when waveguided light strikes theinjection feature114 and is directed out of thewaveguide106. Similarly, light incident on extraction features128 can be effectively injected intowaveguide106. For example, light extracted byextraction feature128 may strikereflective layer136 and be injected back intowaveguide106 by one or more of the extraction features128. Any single ray can interact with a single feature or a number of features and interfaces before finally exiting asoutput light132.
• The exemplary embodiment depicted inFIG. 1 may have a thickness, or depth, that is relatively thin compared with some prior art direct backlights having comparable separation between adjacentlight sources116. In some embodiments, a small cavity or separation betweenwaveguide106 andLCD panel102 is provided to enhance mixing, especially in embodiments that include adiffuser103, described above. In other embodiments, no cavity is necessary since the light is adequately mixed in thewaveguide106. Thedistance142 between thereflective layer136 and theLCD panel102 represents the optical depth of the backlight, including any included diffusers and air gaps, and can be less than thelateral separation140 between adjacentlight sources116, particularly nearest neighboringlight sources116 having similar color characteristics, more preferably less than half thelateral separation140, and even more preferably less than 25 percent of thelateral separation140. This embodiment is generally more readily scalable than edge lit designs, and allows distribution of the heat generated bylight sources116 over a larger area. In another embodiment, the invention takes the form of awaveguide106 for insertion into a conventional direct backlight having anappreciable distance142, for example adistance142 of 0.75 inches or greater, but in this case ahigh transmission diffuser103 can be used in place of a conventional direct backlight diffuser. In yet another embodiment,distance142 is preferably three or more times the thickness ofwaveguide106, with the extra integrating volume containing an air cavity betweenwaveguide106 andhigh transmission diffuser103. Generally, thewaveguide106 is has a locally average thickness that is substantially constant across the viewable region.
• The operation of an exemplary injection feature200 of awaveguide202 is more clearly shown in the cross-section view ofFIG. 2. Thewaveguide202 is a transparent optical material, and in this example, is an acrylic.Injection feature200 is an indentation, generally conical in shape, and filled with a lower index medium such as air. The injection feature200 couples thewaveguide202 to alight source206. The behavior of the light rays generated bylight source206 is based on the refractive indices of theinjection feature200 and thewaveguide202 and the geometry of theinjection feature200. As such, these parameters can be manipulated to enhance the waveguided light within thewaveguide202. Particularly, the parameters can be manipulated to ensure that as much light as possible, preferably a majority of the light, and more preferably substantially all of the light from the light source, is injected and meets the conditions for TIR within thewaveguide202.
• As one example,ray211 from thelight source206 strikes surface220 of theinjection feature200. A resulting, refracted andwaveguided ray212 can be predicted based on theangle224 of theinjection feature200 and the respective refractive indices, which in this case is 1.0 for air and n≈1.49 for acrylic. In order to consider the ray waveguided, and therefore injected into thewaveguide202,ray212 must exceed a certain angle atmajor face203 to be reflected via TIR. TIR occurs when theangle222 betweenray212 and the normal tomajor face203 ofwaveguide202 exceeds a sin(sin(90°)/n), or about 42° in this case. By setting thecone angle224 of the injection feature such that the refractedray212 makes an equivalent angle with theinjection surface220, then any light fromlight source206 entering the injection feature will be injected. This yields acone angle224 of approximately 2*(90−2*42)=12° for acrylic. Theequivalent angle224 for polycarbonate waveguide (n=1.59) would be around 24 degrees, and the cone of theinjection feature200 could be even less steep if higher refractive index materials such as high index glass were used. Larger cone angles224 of theinjection feature200, corresponding to less sharply pointed cones, can also be used if the size of thelight sources206 is smaller than thebase208 of theinjection feature200, or if complete injection of the light is not required. Alternate transmissive materials within the injection features200, such as clear silicone or other adhesives or polymers, with other refractive indices may be used instead of air for better index-matching, with corresponding changes in the refracted rays. Upon reflection,ray212 continues to propagate through thewaveguide202 until theray212 strikes an extraction feature, as discussed in further detail below. In the present embodiment,waveguide202 is a single extended piece, but in other embodiments waveguide202 includes multiple smaller waveguides between which at least a portion of the waveguided rays can pass.
• FIGS. 3-22 depict several exemplary injection features and light sources that can be used in the backlight systems described herein to inject, either primarily or completely via refraction, a majority or more preferably substantially all of the light from the light source such that the light remains confined within the waveguide due to TIR until the light is extracted by an extraction feature. The injection features can be cast, molded, or otherwise formed in or adjacent the waveguide.
• As one example,FIG. 3 is a cross-sectional view of anexemplary injection feature400coupling light source402 to awaveguide404. Theinjection feature400 includes afirst cone406 on a firstmajor face408 of thewaveguide404 and asecond cone410 on a second major face418 of thewaveguide404. The opposing first andsecond cones406 and410 enable a broader cone angle as compared to, for example, the embodiment depicted inFIG. 2. The broader cone angles412 and414 may enable a relativelythinner waveguide404. Thelight source402 is a non-flat LED at least partially extending into thefirst cone406, although in other embodiments, thelight source402 is a flat, side-emitting, Lambertian, or directional LED, or other source having a different source geometry or angular output profile.
• FIG. 4 is a cross-sectional view of anotherexemplary injection feature420coupling light source422 to awaveguide424. In this example, theinjection feature420 is tapered or conical with atruncated end426, which enables a relativelythin waveguide424. To minimize leakage of non-injected light directly out of thewaveguide424, aninsert428, either a specular or diffuse reflector, may optionally be provided within theinjection feature420 at thetruncated end426. In other embodiments, theinsert428 can be white, partially transmissive, adhesive, paint, fill material, an LED cap, and/or a tinted underside.
• FIG. 5 is a cross-sectional view of anotherexemplary injection feature440 coupling light source442 to awaveguide444. In this example, theinjection feature440 is cylindrical with an optional reflector or maskingelement448 at oneend446. The light source442 is a side emitting LED, which may minimize the amount of light that would reach end446 orelement448 prior to injection through theside wall450.
• FIG. 6 is a cross-sectional view of yet anotherexemplary injection feature460coupling light source462 to awaveguide464. In this example, theinjection feature460 is a cylindrical through hole, which is relatively simple to fabricate in that no angular walls within thewaveguide464 are necessary. Amask466, either reflective, scattering, absorbing or a combination thereof is optionally positioned at an end of theinjection feature460 opposite thelight source462. In various embodiments, themask466 can be a white sheet with cutouts, paint, screen printing, tape, adhesive, patterned sheets, and/or partially transmissive or tinted materials.
• FIG. 7 is a cross-sectional view of anotherexemplary injection feature480 coupling light source482 to awaveguide484. In this example, theinjection feature480 is a curved conical shape on amajor face486 of thewaveguide484 opposite the light source482. In various embodiments, theinjection feature480 can be curved, multi-faceted or otherwise complex. Similarly, the simpler conical or cylindrical structures of other embodiments can alternately be curved or multi-faceted as well. While it is preferred for the embodiment ofFIG. 7 that light source482 has a somewhat directional output, this is not required. The light source482 may also include internal side reflectors or other optical mechanisms to assist the directional output.
• FIG. 8 is a cross-sectional view of an exemplary embodiment in which light is directed directly into awaveguide500 by alight source502. In this example, abackscattering layer504, such as white pigment or paint, is provided on amajor face506 opposite thelight source502 to scatter and reflect the light such that a substantial portion of it is injected and waveguided in a lateral direction.Light source502 is preferably a directional light source, although this is not required.
• FIG. 9 is a cross-sectional view of another exemplary embodiment in which light is directed into awaveguide510 by alight source512. In this example, an immersed obliquereflective structure514 serves as the injection feature, injecting the light such that it is waveguided in a lateral direction.
• FIG. 10 is a cross-sectional view of another exemplary embodiment and includes aninjection feature520 that injects light from twolight sources522,524 into awaveguide526. The injection feature520 in this example is a truncated cone, and amasking layer528 is provided to assist theinjection feature520 in injecting the light into thewaveguide526. Themasking layer528 can be applied, for example, by screen printing a diffuse white or specular layer over amajor face530 of thewaveguide526 opposite thelight sources522,524.
• FIG. 11 is a cross-sectional view of another exemplary embodiment and includes aninjection feature540 that injects light from alight source542 into awaveguide544. In this example, theinjection feature540 is cylindrical and thelight source542 is a side-emitting LED. Themasking layer546 can be provided on amajor face548 of thewaveguide544 opposite thelight source542.
• FIG. 12 is a cross-sectional view of another exemplary embodiment and includes aninjection feature560 that injects light from alight source562 into awaveguide564. In this example, theinjection feature560 is cylindrical. Aplug566 is provided in theinjection feature560 opposite thelight source562 to block at least a portion of the light.
• FIG. 13 is a cross-sectional view of another exemplary embodiment and includes aninjection feature580 that injects light from alight source582 into awaveguide584. In this example, theinjection feature580 is cylindrical. Aplug586 can be provided in theinjection feature580 opposite thelight source582 to block at least a portion of the light. In contrast to theplug566 inFIG. 12, theplug586 has beveled or otherwise angled surfaces that may improve injection into the waveguide.
• FIG. 14 is a cross-sectional view of anotherexemplary backlight system620 that includes awaveguide622 coupled to alight source630 by opposingportions632,634 of aninjection feature636. Thewaveguide622 includes afirst substrate624 and a second substrate626. Thefirst substrate624 and the second substrate626 are optically bonded together in a manner such that thesubstrates624 and626 are substantially index-matched, meaning that there is not a low index gap such as an air gap between them. This results in at least a majority of any waveguided light being passed back and forth freely betweensubstrates624 and626. Such bonding can be achieved with optical adhesives or by a variety of other methods, such as thermal, mechanical or chemical processes. Other multiple substrate embodiments comprise one or more other injection feature implementations described above.
• FIG. 15 is a cross-sectional view of anotherexemplary backlight system640. Thebacklight system640 includes awaveguide642 coupled to alight source650. Thewaveguide642 includes afirst substrate644 and asecond substrate646. Thefirst substrate644 may capture at least a portion of any rays from thelight source650 that pass through thesecond substrate646 without being injected. Thewaveguide642 also includes agap648 between the first andsecond substrates644,646, although localized optical bonding or contact can occur in selected locations, using structures similar to those described in reference toFIG. 14. Extraction features, such as those described in referenceFIG. 1 or below, can be arranged in one or both of thesubstrates644,646.
• FIG. 16 is a cross-sectional view of anotherexemplary backlight system660. The backlight system includes awaveguide662 coupled to alight source670. An attenuatingmask layer672 overlays thewaveguide662 to block or attenuate a direct path from thelight source670 out of thewaveguide662.
• FIG. 17 is a cross-sectional view of yet anotherexemplary backlight system680. Thebacklight system680 includes awaveguide682 respectively coupled tolight sources684,686 with injection features688,690. Although still substantially parallel, thewaveguide682 includes wedgedportions692,694 that open upspace696 in between. As a result, thewaveguide682 can have a relatively smaller average thickness and weight and/or the ability to accommodate additional features within thespace696.
• FIG. 18 is a cross-sectional view of another exemplary backlight system940 that includes light sources941-943 coupled to awaveguide944. In this embodiment, light is injected by a combination of individual injection features945-947 and a sharedinjection feature948. Theinjection feature948 has atop portion950 and atapered bottom portion952, which is better shown in the top plan view ofFIG. 19.
• The majority of the injection embodiments thus far have been described in the context of being symmetrical around a vertical axis of symmetry, such as conical or cylindrical. In other embodiments, related structures may have alternate symmetries, for example pyramidal or rectangular, or even be fundamentally asymmetric, for example with an upper portion being slightly offset with respect to the bottom portion. Yet another exemplary symmetry variant is shown inFIG. 20.FIG. 20 is an isometric view of awaveguide800 suitable for use in the backlight systems described herein. Thewaveguide800 includes a plurality of injection features802 and extraction features804. In this embodiment, the injection features802 can accommodate linear light structures such as a fluorescent lamp or rows of LEDs distributed across the viewable region. The use of linear injection features802 or extraction features804 may result in thewaveguide800 having an asymmetric light emitting pattern. In alternate embodiments, injection features802 can accommodate a combination of linear and point light sources such as the LEDs described above. In other embodiments, each of the injection features disclosed above can be extended in a manner such as this, or the various types and orientations of all of the injection features and extraction features may be mixed and matched within or on the waveguide.
• FIG. 21 is a cross-sectional view of anotherexemplary backlight system600. Thebacklight system600 includes awaveguide602 coupled to alight source610. Thewaveguide602 includesflat portions604 and obliquelyangled portions606. Aconical injection feature608 in theangled portion606 couples thewaveguide602 to alight source610. Thewaveguide602 has a generally constant thickness throughout the viewable region and may be thinner than other embodiments, allowing for reduced waveguide weight. The generally constant average thickness improves compatibility with certain manufacturing processes such as compression molding, since removal of material is unnecessary in forming the unitary refractive structure. Only local material flow is required to form the detailed optical surfaces. The embodiment is shown with a flat emitter, but as is the case with the other embodiments, nearly any emitter topology can be utilized. Moreover, any suitable extraction features (not shown) can be used. In another embodiment, theinjection feature608 has the cross-section shown in a first axis, and extends linearly in a second axis, such as was shown forinjection feature802 inFIG. 20. In yet another embodiment,waveguide602 is formed in discrete sections, for example the right and left sides inFIG. 21, which are abutted or joined abovelight source610.
• The geometry of the flat andangled portions604,606 of thewaveguide602 accommodatesadditional circuitry611 between thelight source610 and adjacent light sources (not shown). In some embodiments, all interface and drive circuitry for an LCD system resides on the same plane or board as the one or morelight sources610. Thebacklight system600 further includes a distributedheat sink612 for effectively spreading and removing the heat. Theheat sink612 is correspondingly scalable with thecircuitry611 andlight source610.
• FIG. 22 illustrates a portion of a backlight system that can be used in conjunction with the injection features described above. Particularly,FIG. 21 is a block diagram of exemplarylight source circuitry700 used to drive the light sources of the backlight systems described herein.Light sources762 are grouped into threegroups763,764,765. Eachlight source762 includes adriver772 coupled to anLED774. Eachdriver772 couples to a common signal andpower bus connection770 allowing complex driving and distribution of the supplied current. Thelight sources762 can be dynamically driven as individual LEDs, as groups763-765, or collectively as an entire system. Thedrive circuit772 is optionally contained on a circuit board with theLEDs758 and resides within the lateral gaps betweenadjacent LEDs774. Light spreads uniformly between the groups763-765, which can represent a regular array of sources, distinctly separate source modules injecting light into a larger waveguide, or any other suitable physical layout. This embodiment also enables the suppression of hot spots at theLEDs774 as well as at distinct source modules. In another embodiment, theLEDs774 ingroups763,764,765 are driven as one or more series strings of LEDs.
• In a variation of the embodiment ofFIG. 22, some of theLEDs774 ofFIG. 21 can be replaced with diodes which are non-emissive, or with passive resistive loads. By selectively driving the non-emissive loads to generate heat, the temperature of the backlight can be raised or maintained independently of the brightness setting. This can be useful for maintaining consistent display performance or even for warming a display panel under cold environment conditions, and the effectiveness of the technique is enhanced by the reduced distance and distributed arrangement of the emissive and non-emissive sources of heat. In this variation, either a complex or simplified drive scheme can be utilized. The non-emissive loads can be driven by a separate power source capable of being modulated to adjust the desired rate of heat generation. If desired, thermal conductivity can be included as one of the relevant parameters considered during the process of selecting materials for a corresponding waveguide and other backlight components in order to minimize temperature differences between the display panel and the backlight system.
• WhileFIGS. 4-22 illustrate various types of light sources and injection features,FIGS. 23-27 illustrate several techniques for extracting waveguided light out of the waveguide. For example,FIG. 23 is a cross-sectional view of abacklight system820 having awaveguide822 coupled to alight source824 byinjection feature826, andFIG. 24 is a cross-sectional view of anotherbacklight system840 also having awaveguide842 coupled to alight source844 byinjection feature846. Thebacklight system820 ofFIG. 23 has relatively small extraction features828 as compared to the extraction features848 of thebacklight system840 ofFIG. 24. The relative sizes of the extraction features828,848 result in differences between the spatial extent of the spread function of the light in a lateral direction, which is illustrated by the graph ofFIG. 25, as the larger extraction features848 extract more light than the smaller but comparably spaced extraction features828. In terms of extraction density, each ofbacklight system820 and840 has an extraction density which varies spatially, butbacklight system840 has a generally higher extraction density thanbacklight system820. In these embodiments, this is because the extraction features are larger and more effective while the spacing of the extraction features is comparable.Line850 inFIG. 25 represents the amount of light waveguided and extracted from thewaveguide822 as a function of the distance from thelight source824, andline852 inFIG. 25 represents the amount of light waveguided and extracted from thewaveguide842 as a function of the distance from thelight source844. As such,backlight system820 has a more intense amount of light at and immediately surrounding thelight source824, but thebacklight system840 more evenly distributes light fromlight source844 over a greater area. The effective spread function is therefore determined in large part by the extraction density of the detailed extraction feature design and spacing, and can impact redundancy, color mixing effectiveness, and further topics such as dynamic backlight techniques. The wider spread function represented byline850 is an indication that extraction features828 have a generally lower extraction density than extraction features848 which result in the narrower spread function represented byline852.
• FIG. 26 is a planar view of anexemplary backlight system300 and illustrates the manipulation of the extraction density and the related spread function. In this view, injection features302 are represented by relatively large circles. Extraction features325 are represented by smaller circles. The injection features are arranged in a regular square array with equal separation in both horizontal and vertical directions, e.g.,horizontal distance320 andvertical distance321, but this arrangement is not necessary. The pattern of injection features302 and extraction features325 could be asymmetric rectangular, hexagonal, random, or any other two dimensional array.
• The injection features302 and extraction features325 are arranged intoregions330. Theregions330 are further divided into one ormore subregions340. While distinct subregions are depicted for clarity of explanation, it should be understood that continuously varying distributions of extraction density is a more general case. In the depicted embodiment, thebacklight system300 includes sixteenregions330, each with oneinjection feature302, and eachregion330 includes twenty-fivesubregions340, one of which coincides withinjection feature302. Thesubregions340 are defined by one or more extraction features325, in this case, nine extraction features320. Typically, the extraction features325 of aparticular subregion340 have a particular extraction density. As noted above, extraction density corresponds to the degree by which light is extracted by a particular area of extraction features. The extraction density can be varied, for example, by adjusting the feature density, feature size, feature shape or type of extraction features, and as described previously, facilitates the capability of making the output uniform for a wide variety of LED configurations and waveguide materials. Each of theregions330 andsubregions340 can have varying extraction densities. In this embodiment, the extraction densities of thesubregions340 are manipulated such that light from a respective injection feature302 spreads evenly throughout theregion330, but without significant spread intoadjacent regions330. In other words, the extraction feature topology provides a relatively symmetric and localized spread function.
• In one embodiment, the injection features302 can contribute to an injection leakage density, which is a measure of how much light transmitted directly through the waveguide without being injected. Certain embodiments described herein attempt to minimize the injection leakage density. However, in other embodiments, the extraction density can be tuned to the injection leakage density such that the output light is uniform.
• FIG. 27 is a plan view of anexemplary backlight system860. Thebacklight system860 has a plurality of injection features862 and extraction features864 distributed through awaveguide866. The extraction features864 vary in size throughout the waveguide, as indicated by the relative size of the dots representing the extraction features864. In this embodiment, the topology of the extraction features864 has an asymmetrical design as compared to the more symmetrical design ofFIG. 26. The extraction features864 in an x-direction are relatively constant while the extraction features864 in a y-direction are more varied. As a result, the light injected from injection features862 extends to a greater extent (i.e., a broader spread function) in the x-direction than in the y-direction. This can be particularly useful in applications in which mixing is desired along the x-direction, but not the y-direction, such as dynamic backlighting techniques that synchronize the backlight with the row or column update timing progression or to conserve power or enhance visual contrast by spatially modulating thebacklight system860. In an embodiment of the present invention, an asymmetrical spread function embodiment is combined with the independently dynamic drive embodiment as described inFIG. 22, facilitating dynamically addressable rows of illumination in a reduced depth configuration. In another embodiment, the symmetrical but narrower spread function embodiment ofFIG. 26 are combined with the embodiment ofFIG. 22, facilitating dynamically addressable regions of a compact backlight. In yet another embodiment, a broad spread function is utilized in both x and y directions to facilitate enhanced mixing of light from a plurality of LEDs.
• FIGS. 28-31 illustrate various types of extraction features that can be incorporated in the backlight systems discussed herein. The extraction features can be cast, molded, or otherwise formed.FIG. 28 is cross-sectional view of a portion of abacklight system720 having awaveguide722 with top and bottom major faces724,726 and a plurality of extraction features728-735. Extraction features728,729 are wedge shaped and are formed internal to the bottommajor face726 and topmajor face724, respectively. Extraction features730,731 are irregular and formed on the bottommajor face726 and topmajor face724, respectively. Extraction features732,733 are dimple shaped and formed internally on the bottommajor face726 and the topmajor face724, respectively. Extraction features734,735 are a series of regular wedge shaped or prismatic groove features formed in the bottommajor face726 and the topmajor face724. While only a first cross-section is shown, it should be understood that the other cross-sections can be symmetrical, can be different, or can comprise linear structures as described above. Making the physical cross-sections different, for example slightly broadened in one axis, or the fully extended example ofFIG. 20, allows additional flexibility in achieving uniform output, since the extraction features then have varying angular cross-sections. This can be leveraged to provide extraction densities that depend upon orientation relative to the ray propagation direction as well as spatial location.
• As further examples of extraction features,FIG. 29 is a cross-sectional view of a portion of abacklight system740 having awaveguide742 with top and bottom major faces744,746 and a plurality of extraction features748-756. Extraction features748,749 are wedge shaped and extend from the bottommajor surface746 and the topmajor surface744.Extraction feature750 is an internal, irregular inclusion in thewaveguide742.Extraction feature751 is dimple shaped and extends from the topmajor surface744.Extraction feature752 comprises a localized region in which a diffusely reflectinglayer757, such asreflective layer136 inFIG. 1, is locally index-matched byoptical bond710 tomajor surface746 ofwaveguide742.Optical bond710 can be, for example, a clear adhesive layer such as an applied and cured polymer or a patterned transfer adhesive layer.Extraction feature753 includes an optically structuredupper layer758 which is locally index-matched to topmajor surface744 byoptical bond712. Extraction features754,755 are externally applied diffusing layers on the bottommajor surface746 and the topmajor surface744. A preferred material forextraction feature754 or755 is highly reflective and scattering white paint or a related structure.Extraction feature756 is another example having adiffuser layer759 with a diffuse surface texture and locally index-matched to topmajor surface744 byoptical bond714.
• FIG. 30 is a cross-sectional view of a portion of abacklight system760 with more examples of various types of extraction features764-768.Extraction feature764 is conical shaped and unpolished. Extraction feature766 is more cylindrical thanextraction feature764 and is also unpolished.Extraction feature767 has a profile well-suited to extract light toward a scattering reflector such asreflector136 ofFIG. 1. By keeping the depth of theextraction feature767 small compared with its width, only a small fraction of the extracted and diffusely reflected light will be re-injected through the side walls.Extraction feature768 is an example of a stepped wedge light extraction structure.
• FIG. 31 is a cross-sectional view of a portion of yet anotherbacklight system780. In this embodiment, thewaveguide782 includes atop portion784 and abottom portion786. Thetop portion784 andbottom portion786 are bonded together by an index matching adhesive or other mechanism. Thebottom portion786 can be considered, for example, a secondary layer or film. Injection features788 are arranged throughout thewaveguide782 to couple thewaveguide782 to a light source (not shown), and extraction features790 are arranged to direct light out of thewaveguide782. The injection features788 are formed in both thetop portion784 andbottom portion786 of the waveguide while the extraction features790 are formed on thebottom portion786. Light extracted fromwaveguide782 by extraction features790 may be redirected by a rear reflector, such asreflective layer136 ofFIG. 1.
• FIGS. 32-37 illustrate exemplary backlight systems with light sources having different spectral or color characteristics. For example, full color may be achieved via white light sources (W), a mixing of color light sources such as red, green and blue (RGB), or by mixing both white and colored light sources in varying combinations, such as RW, RBW, RGBW and so forth. Embodiments disclosed herein allow effective mixing of any number or combination of light source contributions while maintaining a low profile or depth and without a substantial extension of components beyond the viewable region. In addition, the number of light sources for each color component can be different, allowing considerable flexibility in adjusting the color gamut, chromaticities or detailed spectral properties of the backlight and resulting display system.
• FIG. 32 is a cross-sectional view of anexemplary backlight system880 that includes a waveguide881, and a plurality of injection features882-885 and extraction features (e.g.,886-891) that respectively inject and then extract light generated by light sources892-895.Light source893 is a red light source, andlight source894 is a blue light source.Light sources892,895 are white light sources. In this example, red and blue light fromlight sources893,894 is injected through the relatively smaller injection features, e.g., injection features883,884. The white light fromlight sources892,895 is injected through the larger injection features, e.g., injection features882,885. Extraction density is varied across the extraction features, for example in the region fromextraction feature890 toextraction feature888, to yield substantially uniform light output in conjunction with any leakage from white light injection, for examplelight source895 andinjection feature885.Injection feature884 in this embodiment is acting as both an injection feature and an extraction feature, allowing colored light to be mixed in with other light in an effective manner.
• FIG. 33 is a cross-sectional view of anotherexemplary backlight system900 that includes awaveguide901, and a plurality of injection features902-907 and extraction features908 that respectively inject and then extract light generated by light sources909-914. In this embodiment, the light sources909-914 can be a combination of various colors, such as red, green, blue and white. The extraction features908 have a lower profile, and in particular a lower extraction density, than some other embodiments to facilitate a broader spread function and an enhanced mixing of the light sources909-914. The spread function in the orthogonal direction may either be comparably broad to facilitate mixing in that direction as well, or may be deliberately shorter as discussed above, and as was seen in the embodiment ofFIG. 27, if color mixing is less important in the second axis. In another embodiment, the multicolor sequence of light sources909-914 can be situated under a linear injection structure, for example injection feature802 ofwaveguide800 inFIG. 20.
• In one embodiment, such as shown in the plan view ofFIG. 34,light sources915 can be grouped intoclusters916 distributed across anactive area region917 of adisplay system918. In a further embodiment, eachcluster916 contains oneLED915 of each color, for example R, G, B and W. In yet a further embodiment, eachcluster916 is injected into a waveguide such aswaveguide526 ofFIG. 10 viainjection feature520 ofFIG. 10.
• FIG. 35 is a cross-sectional view of anotherexemplary backlight system920 that includes awaveguide921. Injection features922 and extraction features923 respectively inject and then extract light generated bylight sources924. Thebacklight system920 further includes additionallight sources925 that inject light through one ormore edges926 of thewaveguide921.
• FIG. 36 is a cross-sectional view of anotherexemplary backlight system930 that includes awaveguide931. Injection features932 and extraction features933 respectively inject and then extract light generated bylight sources934. Thebacklight system930 further includeslight absorbing features935 to tune a particular portion of spectrum and chromaticity of thebacklight system930 without introducing uniformity concerns.
• While at least one exemplary embodiment has been presented in the foregoing detailed description of the invention, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention. It being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims.

Claims (20)

1. A backlight system for a liquid crystal display, comprising:

a substantially planar, refractive waveguide including a first major face and a second major face opposite the first major face, the waveguide including a viewable region corresponding to a viewable area of the liquid crystal display;

a light source positioned proximate to the second major face and within the viewing region for producing light;

an injection feature proximate to one or more of the second major face and the first major face and within the viewing region to optically couple the light into the waveguide such that the light becomes waveguided light; and

a plurality of extraction features proximate to one or more of the second major face and the first major face and within the viewing region to optically couple the waveguided light out of the waveguide.

2. The backlight system ofclaim 1, wherein a plurality of the light sources and a plurality of the injection features are positioned within the viewable region, the plurality of the injection features being interspersed amongst the plurality of the extraction features.

3. The backlight system ofclaim 2, wherein the plurality of light sources contains light sources having different color characteristics, and wherein the light of different color characteristics are at least partially mixed within the waveguide.

4. The backlight system ofclaim 1, wherein at least a portion of the waveguided light is optically coupled directly from the injection feature to out of the waveguide without leaving the viewable region.

5. The backlight system ofclaim 1, wherein substantially all of the light from the light source is optically coupled into the waveguide such that it becomes waveguided light.

6. The backlight system ofclaim 1, wherein the injection feature is a unitary structure.

7. The backlight system ofclaim 1, wherein the waveguide is a unitary refractive waveguide.

8. The backlight system ofclaim 1, wherein the injection feature includes a first portion on the first major face and a second portion on the second major face.

9. The backlight system ofclaim 1, the backlight system having an optical depth and further comprising

a reflective layer proximate the second major face;

a transmissive diffuser proximate the first major face; and

at least one additional light source having a similar color characteristic to the first light source, wherein the optical depth is less than a lateral separation between the light source and the at least one additional light source.

10. The backlight system ofclaim 1, wherein the waveguide has a locally averaged thickness that is substantially constant across the viewable region.

11. A backlight system for a liquid crystal display, comprising:

a substantially planar waveguide including a first major face and a second major face opposite the first major face, the waveguide including a viewable region corresponding to a viewable area of the liquid crystal display;

a light source positioned proximate to the second major face and within the viewing region for producing light;

an injection feature proximate to at least one of the second major face and the first major face and within the viewing region to optically couple the light into the waveguide such that the light becomes waveguided light; and

a plurality of extraction features proximate to at least one of the second major face and the first major face and within the viewing region to optically couple the waveguided light out of the waveguide, the plurality of extraction features having an extraction density that varies.

12. The backlight system ofclaim 11, wherein the injection feature has an injection feature leakage that matches the extraction density.

13. The backlight system ofclaim 11, wherein the extraction features have a first extraction density in a first direction from the injection feature and a second extraction density in a second direction from the injection feature.

14. The backlight system ofclaim 11, wherein the waveguide has a first spread function in a first direction and a second spread function in a second direction, the first and second spread functions being different.

15. The backlight system ofclaim 11, the backlight system having an optical depth and further comprising

a reflective layer proximate the second major face;

a transmissive diffuser proximate the first major face; and

at least one additional light source having a similar color characteristic to the first light source, wherein the optical depth is less than a lateral separation between the light source and the at least one additional light source.

16. The backlight system ofclaim 11, the backlight system having an optical depth and further comprising

a reflective layer proximate the second major face; and

a transmissive diffuser proximate the first major face, wherein the optical depth is at least three times a distance between the first major face and the second major face.

17. The backlight system ofclaim 11, wherein the extraction density of at least a portion of the extraction features varies with one or more of orientation and spatial position.

18. The backlight system ofclaim 11, wherein the light source is a first light source, the injection feature is a first injection feature, and the backlight system further comprises a second light source and a second injection feature which optically couples light from the second light source into the waveguide such that it becomes waveguided light, and wherein the second injection feature is one of the extraction features that optically couple the waveguided light from the first light source out of the waveguide.

19. The backlight system ofclaim 11, wherein the light source is a first light source and the backlight system further includes a second light source and circuitry configured to individually drive the first and second light sources.

20. A liquid crystal display (LCD), comprising:

an LCD panel having a plurality of pixels; and

a backlight system coupled to and illuminating the pixels to form an image, the backlight system including

a substantially planar dielectric waveguide including a first major face and a second major face opposite the first major face, the waveguide including a viewable region corresponding to a viewable area of the liquid crystal display;

a light source positioned proximate to the second major face and within the viewing region for producing light;

an injection feature within the viewing region to optically couple the light into the waveguide via refraction such that the light becomes waveguided light; and

a plurality of extraction features within the viewing region that optically couple the waveguided light out of the waveguide, the plurality of extraction features having an extraction density that varies.

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